Trump has been openly warring with the entirety of the GOP presidential field — with the exception of Cruz — making the point that he’ll only go on the attack if he’s hit first.

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Cruz, meanwhile, has been drafting behind Trump, openly acknowledging that it would be poor strategy for him to bash a candidate whose supporters he seeks and accusing the media of trying to arrange a “cage fight” between the two.

But there was movement on Cruz’s end Thursday, when The New York Times reported that Cruz questioned Trump’s judgment in front of donors at a closed-door fundraiser in Manhattan.

It was a soft jab that Cruz sought to soften further by calling the Times report “misleading.”

Still, Cruz did not challenge the central facts of the story, making it the furthest he’s gone toward criticizing Trump to date.

Republicans say Cruz has likely opened the door to a furious response from the Trump, the unpredictable showman who has been itching for a fight.

“Trump’s strategy has been to generate controversy, tout his standing in the polls, and go on the attack,” said Republican strategist Ford O’Connell. “With Cruz gaining on him, you can bet Trump will go after him as soon as he moves past this latest firestorm over not allowing Muslims into the country.”

The Trump campaign is not yet tipping its hand.

Spokeswoman Katrina Pierson said on MSNBC on Thursday that it was her understanding that Cruz had walked his remarks back.

“I just really don’t know,” Pierson said. “Mr. Trump will be the one to make that decision. If he feels that Mr. Cruz is going after him, he’ll probably respond ... but only Mr. Trump speaks for Mr. Trump.”

Still, a ramping-up of rhetoric between the two Iowa front-runners feels imminent, whether at a campaign rally, where Trump delights in knocking down his opponents one-by-one, or at next week’s GOP debate, where they’ll square off for the first time since early November.

“Cruz has been trying to appeal to Trump supporters on the chance that he’ll be around to pick up the pieces if Trump fails,” said Jeffrey Lord, a former Reagan administration official and Trump supporter. “At this point, I think he’s realized that Donald is not going to implode so he’s making his move. This is what presidential primaries are about. This is what they do. It’s time they draw distinctions.”

Trump has said repeatedly at rallies that he expects Cruz will have to break his vow not to attack him. He has promised to respond forcefully once it happens.

But regardless of whether Trump believes Cruz went after him on Thursday, Republican senator has already passed one of Trump’s thresholds for attack.

A Monmouth University survey released this week found Cruz passing Trump in Iowa, surging to a 5-point lead over the longtime front-runner.

But according the RealClearPolitics average of polling, Trump still holds the lead overall, after beating back a months-long surge by Ben Carson, though Cruz has closed the gap, trailing by 3.4 points.

Cruz has stolen much of Carson’s evangelical support in the state, polls find, as his persistent courting of the Christian community in the state has paid off. On Thursday, Cruz landed the endorsement of influential Iowa conservative leader Bob Vander Plaats.

Among Republicans, the only question that remains is how these two candidates, both with anti-establishment appeal, plan to come at one another with less than two months before the caucuses are held in the Hawkeye State.

For Trump, the possible angles of attack are endless, as Republicans say nothing has proven off-limits.

“Could he attack Cruz from the 'birther' angle because he was born in Canada? Maybe point out that he’s a first-term senator like Obama? All of that is possible,” said Republican strategist Matt Mackowiak, who writes for The Hill's Contributor's blog.

Trump may also have a few policy jabs up his sleeve, having staked out the far right on immigration, with his call to deport all illegal immigrants, and terror, with his proposal to ban Muslims from entering the country.

Cruz’s bag of tricks may be smaller, as he’s still the candidate in pursuit and won’t want to alienate Trump’s supporters.

To date, Cruz has carefully chosen his spots and sought to draw subtle policy distinctions between himself and Trump. He has repeatedly declined to criticize Trump's incendiary rhetoric or attack him personally.

Republicans still don’t expect Cruz will go all-in against Trump, the way contenders such as Jeb Bush, John Kasich, Rick Perry and Bobby Jindal have.

They say he’ll continue to look for areas of policy disagreement, perhaps on matters related to Trump’s past support for universal healthcare or eminent domain, or over his proposed Muslim ban.

But Republicans say Cruz’s “judgment” jab could also be effective by framing the showman as unpredictable and erratic.

“A lot of Republicans view Trump as strong on national security issues and believe he’s a strong leader,” said O’Connell. “So by questioning his judgment, Cruz is subtly pointing out that Trump is erratic and liable to fly off the handle, which isn’t a quality people like in a leader.”

However it unfolds, Republicans see peril for Cruz if he doesn’t play his cards right.

“I like Cruz,” said Lord. “But we’ve seen what happens when you go after Donald wholesale. It backfires every time.”